Fix wamr compiler issues and refine some error messages (#470)

Fix potential memory leak issue when using llvm::EngineBuilder().selectTarget()
Fix issue of accessing aot_value's fields after it is freed
Fix JIT not print failed to link import warning
Change some error messages: 'fail to' to 'failed to'
Update error message when SIMD isn't enabled
Fix install littlevgl wasm app of wasi version failed

Signed-off-by: Wenyong Huang <wenyong.huang@intel.com>

samples/littlevgl/wasm-apps/src/main.c

@@ -159,6 +159,13 @@ static void hal_init(void)
     indev_drv.type = LV_INDEV_TYPE_POINTER;
     indev_drv.read = display_input_read; /*This function will be called periodically (by the library) to get the mouse position and state*/
     lv_indev_t * mouse_indev = lv_indev_drv_register(&indev_drv);
-
+}
+
+/* Implement empry main function as wasi start function calls it */
+int main(int argc, char **argv)
+{
+    (void)argc;
+    (void)argv;
+    return 0;
 }